The present invention relates to a system for tuning a receiver antenna circuit and amplifier to maximize the signal/noise ratio and, more particularly, to a system for tuning a receiver antenna circuit and amplifier in a Magnetic Resonance Imaging (MRI) system.
In a typical MRI system, the signal voltage is produced by a resonant antenna circuit that drives a low-noise amplifier. The antenna senses the rotating magnetization inside a test sample, such as a patient, from a nuclear magnetic resonance (NMR) procedure. Typically, the rotation frequency is in the high frequency range of between approximately one to 100 MHz. The frequency depends on the magnetic field strength in the MRI system.
The antenna circuit in an MRI system typically contains a tuning adjustment, such as a variable capacitor. For example, a resonant antenna can be a simple coil, or inductor, surrounding or adjacent the sample to be tested. The antenna is tuned by an external capacitor which forms an "RLC" resonant circuit.
FIG. 1 illustrates such a circuit, having inductance L, capacitance C and loss resistance R. The rotating magnetization induces an electromotive force (emf) E in series with the inductance L and the voltage across the capacitance C drives the amplifier. The circuit will resonate at the operating frequency f.sub.o when the capacitor is adjusted to satisfy equation 1 given as: EQU 1/.omega..sub.o C)=.omega..sub.o L where .omega..sub.o =2.pi..times.f.sub.o (1)
This adjustment is called tuning.
The loss resistance R shown in FIG. 1 is the actual resistance of the circuit elements, inductor L and capacitance C, and the loss coupled from the conductivity and dielectric loss of the test sample located within the field of the coil. In the circuit of FIG. 1, the signal is produced by the emf E and the noise is produced by the thermal noise e.sub.R in resistance R at room temperature. The input signal to the amplifier is maximized approximately when the circuit is tuned to resonate as in Equation 1.
To maximize the signal/noise ratio of the system, the loss in the MRI receiver antenna must be minimized. The loss of the receiver antenna is defined as the dissipation as heat of the energy stored in the resonant circuit. This heat dissipation is lost energy to the receiver system. Some of the loss comes from the current induced in the test sample, and this loss cannot be reduced without reducing the volume of the test sample coupled to the coil. The remaining loss of the system is produced from the wire in the coil and the unwanted resistance of the tuning capacitors and other components that form the total antenna circuit. This loss is expressed in terms of a "quality factor" (Q). Q is inversely related to the loss; a high Q resonant circuit dissipates very little of its stored energy in one cycle of the oscillating current. Moreover, the Q determines the bandwidth of the resonant circuit. A high Q resonant circuit has a narrow bandwidth. If the Q of the tuned circuit is high, the difference between the maximum output and the resonant tuning conditions is negligible. Therefore, the tuning condition results in the maximum output from the amplifier.
In the circuit of FIG. 1, the noise generator of the resistance R is in series with the signal generator E, and thus, the noise voltage input to the amplifier will see the same gain factor as the signal voltage. Therefore, the signal/noise ratio will not change with the tuning adjustment because there is no amplifier noise to consider in this circuit. The circuit is tuned merely to maximize the signal level of the amplifier.
FIG. 2 illustrates a noisy amplifier used with the RLC tuned circuit. The amplifier noise is shown as noise voltage generator e.sub.n having a constant magnitude. The amplifier noise e.sub.n is in series with the high-impedance amplifier input. In this case, to maximize the signal/noise ratio, the signal level at the amplifier input must be maximized. The signal/noise ratio of the FIG. 2 circuit is maximized when equation 1 is satisfied by adjusting capacitor C, which also gives maximum signal output from the amplifier.
Capacitor C is adjusted either manually or automatically by computer software. The procedure uses either a constant signal coupled to the antenna from a signal generator at the operating frequency or a repeated NMR signal from the actual MRI system. If the noise voltage e.sub.n of the amplifier is very small, the signal/noise ratio will be a weak function of the tuning and precision in the adjustment of the capacitor is not necessary.
FIG. 3 illustrates a more realistic model of a noisy amplifier connected to an RLC resonant circuit. The amplifier has input noise voltage, e.sub.n, input noise current, i.sub.n, and input impedance, R.sub.in. In this model, the input impedance R.sub.in is not a noise source. All of the noise generated by the amplifier in this model is produced by the two generators e.sub.n and i.sub.n. This circuit differs from the circuit of FIG. 2 in that the amplifier noise generated in the FIG. 3 circuit is affected by tuning.
The contribution of the noise current generator to the total noise voltage is a function of the source impedance seen by the amplifier terminals. The amplifier input impedance also affects the quality factor, Q, of the tuned circuit. If the amplifier input impedance is made very low there will be a large difference between the tuning condition for the maximum signal level of the amplifier and the tuning condition for the best signal/noise ratio. This tuning shift can have a great effect on the signal/noise ratio if the amplifier input impedance results in a large reduction of the resonant circuit Q.
FIG. 4 illustrates a commonly used method to tune a receiver amplifier for the best signal/noise ratio. The tuning capacitor C of FIG. 3 is replaced by a more complicated capacitor network that has two separate adjustments, C.sub.tune and C.sub.match. With these two degrees of freedom, the output impedance of the resonant network at the connection port can be adjusted over a wide range of complex values, and is typically adjusted to 50 ohms (real) at the operating frequency f.sub.o. The amplifier connected to this port is pre-adjusted to give the best signal/noise ratio from a 50 ohm source impedance. To tune this system, C.sub.tune and C.sub.match are adjusted to give 50 ohms (real) at the connection port, as measured with a suitable impedance meter. After this adjustment is made, the connection port is connected to drive the amplifier. These adjustments do not necessarily result in the maximum signal at the amplifier output, but they do result in the best signal/noise ratio.
A problem exists with this method of tuning for best signal/noise ratio because it requires a 50 ohm system and an external impedance meter. Furthermore, the connection port must be changed or switched between the tuning adjustment and actual operation of the receiver antenna. At this very sensitive point in the circuit, connectors and switches can be unreliable, and may introduce noise or interference into the circuit.
In some receiver systems, the input impedance of the amplifier is adjusted with feedback to obtain a desired value of Q for the resonant circuit. This is known as "Q damping". Q damping requires a direct connection between the tuned circuit and the amplifier, and it is not usually possible to transform the impedance at the connector port to 50 ohms. If the desired Q value is very low, to reduce the effect of coupling between several tuned circuits, the tuning shift between maximum signal level of the amplifier and best signal/noise condition may be substantial and the signal/noise ratio may be too low when the antenna circuit is tuned for the maximum signal level.